Determination of Trace Amounts of Molybdenum

Molybdenum is determined directly in solutions containing above 100 ppb. A spectrophotometric method using the extractable complex with diacetyldithiol was found to be the most convenient and the least subject to interference of a number of methods.

Preconcentration of dissolved molybdenum by coprecipitation with hydrated MnO2 followed by spectrophotometry of the dithiol complex allows the determination of a 1 μg of molybdenum in a 5 liter sample (0.2 ppb) with a relative error of 3 percent.

Particulate matter may be removed from water samples by HA Millipore filtration or by continuous flow (150 ml/min) centrifugation at 28,000 g. The latter method is more convenient for processing the large (10L to 50L) water samples needed for estimating the trace metal content of suspended particles in unpolluted waters.

Molybdenum is most conveniently released from organic materials by wet oxidation with perchloric acid, the residue being taken up in 3-N hydrochloric acid. Hydrofluoric acid digests of silicate samples were found to give higher recoveries than perchloric acid digests and were found to be more reproducible than fusion methods.